Turnip

NC State Extension, 'Clubroot of Brassicas', content.ces.ncsu.edu/clubroot-of-brassicas (read 2026-07-11): 'Plasmodiophora brassicae is capable of surviving in the soil for more than 10 years by producing thick-walled, resting spores'; 'Infested fields should be rotated away from brassica for a minimum of 4-7 years, but this is not guaranteed to prevent disease'; 'Liming to raise soil pH to 7.[2]'.

Cornell Vegetables, 'Clubroot of Crucifers', vegetables.cornell.edu/pest-management/disease-factsheets/clubroot-of-crucifers (read 2026-07-11): 'the pathogen survives for many years even in the absence of host crops'; 'rotation of at least 7 years out of susceptible crucifers may be effective.'

UMass Extension, 'Brassicas, Clubroot', ag.umass.edu/vegetable/fact-sheets/brassicas-clubroot (read 2026-07-11): 'Long (5-7 years) rotations between brassica crops'; 'Maintain a high pH by regular applications of lime.'

UMN Extension, 'Growing carrots and parsnips in home gardens', extension.umn.edu/vegetables/growing-carrots-and-parsnips (read 2026-07-11): 'Excessive nitrogen fertilization can also contribute to lots of leaf growth at the expense of root growth'; 'Forked, or split, roots may result from rocky, heavy, or compacted soil.'

Univ. of Illinois Extension, 'Carrots', extension.illinois.edu/gardening/carrots (read 2026-07-11): 'excess organic debris such as woodchips worked into the soil just before planting also may affect root penetration, causing forked and twisted roots'; hairy roots 'are caused by a viral disease known as "aster yellows."'

NC State Extension, 'Clubroot of Brassicas', content.ces.ncsu.edu/clubroot-of-brassicas (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'The pathogen germinates in wet soils when the soil temperature is between 18 and 25 ºC (64 to 77 F), and the soil pH is acidic (pH less than 7.0)'; 'Alkaline soils and soils with high calcium content suppress the potential of the pathogen to initiate infection'; 'Liming to raise soil pH to 7.1-7.5 can slowly reduce disease severity, although this strategy is not completely reliable when there is a well-established infestation'; 'Calcitic lime is recommended over dolomitic lime unless the field has low levels of magnesium'; host list 'broccoli, Brussels sprouts, cabbage, Chinese cabbage, cauliflower, collard, kale, kohlrabi, mustard, oilseed rape (canola), radish, rutabaga, and turnip.'

UMass Extension, 'Brassicas, Clubroot', umass.edu/agriculture-food-environment/vegetable/fact-sheets/brassicas-clubroot (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'The disease is favored by soils with a pH less than 7.0'; 'Maintain a high pH by regular applications of lime'; 'Calcitic lime is preferred over dolomitic, except where magnesium levels are low'; 'High pH can lead to boron deficiency in coarse soils'; 'occurs on broccoli, Brussels sprouts, cabbage, cauliflower, turnip, rutabaga, and radish.'

UConn IPM, 'Clubroot of Crucifers', ipm.cahnr.uconn.edu/clubroot-of-crucifers (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Maintain soil pH at 7.0-7.2 where clubroot is a concern. Lime well ahead of planting and incorporate thoroughly'; 'Monitor boron where pH is raised, especially on coarse soils'; 'It affects crops such as cabbage, broccoli, cauliflower, Brussels sprouts, kale, collards, turnips, rutabagas, radishes, mustard greens, and other brassicas.'

Cornell Vegetables, 'Clubroot of Crucifers', vegetables.cornell.edu/pest-management/disease-factsheets/clubroot-of-crucifers (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): host range - most susceptible 'cabbage, Chinese cabbage, Brussels sprouts, some turnips, wormseed mustard, and some species of candytuft'; medium 'kohlrabi, kale, cauliflower, collards, broccoli, rutabaga, seakale, some turnips and radishes'; mildly 'rape (canola), black mustard, some turnip and radish varieties, and tumble mustard'; very resistant or occasionally immune 'wintercress; or yellow rocket, horseradish, shepherd's purse, wallflower, dames violet, peppergrass, garden cress, stock, and some radishes'. On pH: 'soil pH should be adjusted to pH 6.8 with ground limestone'; 'It is essential that hydrated lime be added each year unless the soil pH exceeds 7.5'; 'the use of nitrogen fertilizers will reduce soil pH (nullifying the beneficial effects of liming).'

University of Illinois Extension, 'Time Your Vegetable Plantings by Cold Hardiness' (2018-03-26), extension.illinois.edu/blogs/ilriverhort/2018-03-26-time-your-vegetable-plantings-cold-hardiness: very hardy vegetables 'withstand freezing temperatures and hard frosts without injury' (planted earliest in spring); frost-tolerant vegetables 'can withstand light frosts, but not freezing temperatures' (planted a couple of weeks later). LLM-summarised from a read of the page; it establishes the CATEGORIES but states no week offsets.

Colorado State Univ. Extension, GardenNotes #720, 'Vegetable Planting Guide', extension.colostate.edu/resource/vegetable-planting-guide (read 2026-07-25): hardy vegetables go in 'As early as 2-4 weeks before the date of the average last spring frost'; semi-hardy 'As early as 0-2 weeks before the date of the average last spring frost'; tender from seed 'around the date of the average last spring frost'; very tender 'Typically 2+ weeks after'. This is the source that states the offsets in weeks.

Where this rule sits in that range: hardy at 3 weeks is mid-range of CSU's 2-4. Half-hardy at 2 weeks is the EARLIEST EDGE of CSU's 0-2 - the most aggressive planting CSU sanctions, not the middle of it. The rule is inside published guidance at both ends, but it leans early on the half-hardy side and a gardener following it in a cold spring is taking that boundary.

Do not import an offset from another publication's hardiness scheme without checking the category names: CSU has no 'very hardy' tier, so a '4-6 weeks before frost' figure written against a four-tier scheme does not describe CSU's 'hardy'. Categories are not comparable across sources.

UMN Extension, 'Growing cool-season crops - non-pest issues', extension.umn.edu/planting-and-growing-guides/non-pest-issues-cool-season-crops (read 2026-08-21): 'Bolting refers to premature flowering. It occurs at any point after a plant has shifted into its reproductive stage'; 'Since bolting is partially triggered by the accumulation of cold, these strategies can reduce exposure to cold temperatures for young plants'; 'the ideal situation for cool-season vegetables is cool but moderate temperatures (50s and 60s) for the first month of growth'; 'when spring weather is quite cool, plants can experience vernalization early, and the result is buttoning up, or the formation of small, often misshapen heads.'

UMD Extension, 'Flower Stalks Form or Bolting of Vegetables', extension.umd.edu/resource/flower-stalks-form-or-bolting-vegetables (read 2026-08-21): 'Biennial vegetable crops complete their life cycle in 2 years. But many will produce flower stalks in response to cool or hot temperatures and long days'; 'Onion-biennial seed stalks form if weather is cool early in its growth cycle.'